With the recent discoveries of new families of DNA polymerases, it has become clear that all living organisms possess many more DNA polymerases than are needed for simple replication of DNA on leading and lagging strands. Humans possess at least 16 DNA polymerases and even the simple bacterium Escherichia colt has 5 currently known. Many of the newly-discovered DNA polymerases carry out specialized functions that allow cell survival in response to environmental challenges. The translesion polymerases allow the replisome to traverse specific DNA lesions that otherwise block normal replicative polymerases, allowing survival when lesions exceed DNA-repair capacity. In doing this, some translesion polymerases introduce mutations, and many are low-fidelity polymerases that are highly mutagenic on lesion-free DNA. Thus, use of these low-fidelity polymerases appears to entail a trade-off between immediate survival of environmental insults and mutagenesis, which can lead to carcinogenesis in eukaryotes, and potentially to reduced fitness in microbes. The choice of which DNA polymerase is used at the replisome is therefore a critical cellular decision. A key player in polymerase choice is the replicative sliding-clamp proteins (PCNA in eukaryotes and beta, encoded by DNAN, in E. coli). This project uses a novel assay for determining which parts of, and how, the beta clamp mediates competition of the 5 E. coli DNA polymerases, and functions in DNA polymerase choice in vivo: a critical regulatory decision for cell survival of environmental damage, and genomic health. [unreadable] [unreadable] [unreadable]